One of the key components of a computer system is a place to store data. Typically computer systems employ a number of storage means to store data for use by a typical computer system. One of the places where a computer can store data is in a disk drive which is also called a direct access storage device.
A disk drive or direct access storage device includes several disks which look similar to 45 rpm records used on a record player or compact disks which are used in a CD player. The disks are stacked on a spindle, much like several 45 rpm records awaiting to be played. In a disk drive, however, the disks are mounted to the spindle and spaced apart so that the separate disks do not touch each other.
The surface of each disk is uniform in appearance. However, in actuality, each of the surfaces is divided into portions where data is stored. There are a number of tracks situated in concentric circles like rings on a tree. Compact disks have tracks as do the disks in a disk drive. The tracks in either the disk drive or the compact disk essentially replace the grooves in a 45 rpm record. Each track in a disk drive is further subdivided into a number of sectors which is essentially just one section of the circumferential track.
Disks in a disk drive are made of a variety of materials. Most commonly, the disk is made of metal or plastic. The material from which the disk is made determines how data is stored on the disk. A plastic disk, such as those used as CD's, stores data using lasers and a laser is used to read the data back. Storage of data on a metal disk entails magnetizing portions of the disk in a pattern which reflects the data.
To store data on a metal disk, the metal disk is magnetized. In order to magnetize the surface of a disk, a small ceramic block which contains a magnetic transducer known as a read/write head is passed over the surface of the disk. More specifically, the read/write head is flown at a height of approximately six millionths of an inch from the surface of the disk and is flown over the track as the read/write head is energized to various states causing the track below to be magnetized to represent the data to be stored.
To retrieve data stored on a magnetic disk, the read/write head is flown over the metal disk. The magnetized portions of the disk induce a current in the read/write head. By looking at output from the read/write head, the data can be reconstructed and then used by the computer system.
Like a record, both sides of a disk are generally used to store data or other information necessary for the operation of the disk drive. Since the disks are held in a stack and are spaced apart from one another, both the top and the bottom surface of each disk in the stack of disks has its own read/write head. This would be comparable to having a stereo that could play both sides of a record at once. Each side would have a stylus which played the particular side of the record.
Disk drives also have something that compares to the tone arm of a stereo record player. There are two types of disk drives, rotary and linear. Rotary disk drives have a tone arm that rotates much like a record player. The tone arm of a rotary disk drive, termed an actuator arm, holds all the transducers or read/write heads, one head for each surface of each disk supported in a structure that looks like a comb. The structure is also commonly called an E block. Like a tone arm, the actuator arms rotate so that the read/write heads attached to the actuator arm can be moved to locations over various tracks on the disk. In this way, the read/write heads can be used to magnetize the surface of the disk in a pattern representing the data at one of several track locations or used to detect the magnetized pattern on one of the tracks of a disk. For example, the needed data may be stored on two different tracks on one particular disk, so to read the magnetic representations of data, the actuator arm is rotated from one track to another track. A linear actuator has a similar actuator arm, however, instead of repositioning by rotation, repositioning is accomplished through linear movement. This particular invention deals with minimizing the time necessary for repositioning the actuator arm.
Increasing the speed at which data can be retrieved is very desirable in a disk drive or direct access storage device. This is due mainly to the fact that decreasing the access time increases the speed at which a computer system can handle data. Since data can be handled more quickly, more transactions can generally be handled by a computer in a particular unit of time.
Attached to the actuator arm is a coil, most commonly known as a voice coil. The voice coil is one of the major portions of an electrical motor, known as the voice coil motor, which is used to move the actuator arm. By controlling the amount and direction of the current passing through the voice coil, the direction and the speed at which the actuator arm can be moved is regulated. Of course, the voice coil does have physical constraints one of which is a maximum current which can be passed through the voice coil for a given supply voltage. When maximum current is passed through the voice coil this is called operating in saturation mode. The amount of current applied to the voice coil during saturation mode is referred to as the saturation current.
Most of the methods for controlling access time include referring to a velocity profile. A velocity profile is a pre-programmed equation or table which lists a desired velocity verses the stopping distance remaining until reaching the target track. The profile velocity value is the highest possible value of velocity the actuator can have at a particular distance so that the actuator can still be decelerated to a stop upon reaching the target track. The amount of deceleration that can be applied to the actuator is a function of many variables including voice coil resistance, file torque constant and power supply voltage. These variables are generally not known for each specific file and as a result, the velocity profile is designed using worst case values to assure that there will always be adequate deceleration capability to stop the actuator upon reaching the target track.
A typical seek is accomplished by calculating distance left to go to the target track, selecting the velocity from the velocity profile which corresponds to the calculated distance to go, determining the actual actuator velocity and subtracting the actual actuator velocity from the selected velocity obtained from the velocity profile. This value is then multiplied by a gain to give a control current output to the voice coil. This method is well known in the art as the closed loop control method.
When the profile velocity is larger than the actual velocity, the result of subtracting actual actuator velocity from the selected velocity obtained from the velocity profile is positive, and the actuator is accelerated. When the profile velocity is less than actual velocity, the result of subtracting actual actuator velocity from the selected velocity from the velocity profile is negative, and the actuator is decelerated. The gain is chosen in the closed loop control method so that it is as high as possible yet still within the limits of stability and such that good conformity to the velocity profile is achieved.
The prior methods have several disadvantages. One of the disadvantages is that the access time or the time necessary to move the actuator arm so the magnetic transducers attached to it can be relocated over another portion of one of the disks, is not minimized. This is due in part to the prior methods tracking the velocity profile of the particular disk file as closely as possible during acceleration and deceleration of the actuator arm. Since the velocity profile is designed assuming worst case conditions so that adequate deceleration is available, then all of the files not operating under worst case conditions operate at less than an optimal level.